It seems to me that the claims, in the existing answers and comments, that a mention in the acknowledgements never matters, are coming from the perspective of academics who rarely, if ever, cross paths with the hiring of laboratory technicians. I have not done so either, but I would put a hold on that claim until someone with that direct experience does.
In the meantime, though I would advance the claim that a CV entry of the form
Worked in the X laboratory doing Y, which contributed by doing Z to the research reported in the paper W
is a perfectly reasonable way to describe the activity - it's not authorship, but it still accurately describes the work without claiming it - and that if the paper contains an explicit mention in the acknowledgements then that will definitely shore up the claim.
I don't know how this would actually play in a postgraduate admissions panel in a field where research-assistant experience would be considered as a valuable asset, but I wouldn't discount its value without hearing from someone with knowledge of that situation.
(Apologies for the tangential answer, but it seemed preferable to multiple identical replies. As to how you get the acknowledgement to happen, I would recommend a light-touch personal interaction to let them know that you feel that it does impact your future career, and then see how that goes.)